The reaction by which formaldehyde is obtained from methanol by catalytic oxidation has been known since 1868 and the use of silver catalysts for this purpose has been known at least since 1908 as is disclosed in German Pat. No. 228,687. Though other catalytic metals and metal oxides have been proposed and used, the use of silver for this process is quite widespread.
The basic method for carrying out the reaction comprises oxidizing an air-methanol mixture over silver catalyst and then recovering the formaldehyde by condensation and absorption with water. It is usually neccessary that the methanol content of the product be no greater than about 2% by weight in a 56% by weight aqueous solution of formaldehyde.
Silver-catalyzed processes for making formaldehyde from methanol can be characterized according to the number of catalytic stages used to effect the conversion. Single stage operation is quite widely used but suffers from the disadvantage that rather high amounts of unconverted methanol are contained in the product emerging from the catalyst bed. This phenomenon is customarily referred to as "methanol leakage". Since for many applications methanol is an undesirable contaminant, it must be separated from the formaldehyde solution. This entails a substantial investment is distillation facilities and energy to carry out such separations.
One way of reducing the need for facilities to distill off methanol is to use two oxidation stages with interstage cooling. A basic two-stage process of this type was disclosed in U.S. Pat. No. 2,462,413 to Meath. In Northeimer's allowed U.S. Pat. application Ser. No. 448,994, filed Mar. 7, 1974, an improvement on the Meath process is disclosed by which even lower amounts of methanol in the absorber product can be obtained.
A widely used catalyst for the manufacture of formaldehyde by oxidation of methanol is foraminous silver in the form of sintered mesh, gauze or crystals, particularly crystals prepared by electrolytic deposition from aqueous acid solutions of silver salts, such as silver nitrate. Such catalytic materials become sintered in the course of being heated to temperatures of from about 550 to about 700.degree. C, to which they are normally exposed during adiabatic operation of the oxidation reaction.
Particularly in two-stage operations, it has been found preferable to employ finely divided silver crystals rather than silver gauze in the second oxidation stage in order to reduce methanol leakage to a satisfactory level while retaining satisfactory yields and conversion. The catalyst must, of course, give low methanol leakage throughout operation of the process. However, this is very difficult during the first several hours to several days of operation of the process. During this time methanol leakage is highest and is reduce to a relatively uniform lower volume at which it remains during the remainder of the life of the catalyst if operated at equivalent conditions. Thus, during the startup and first few days of operation, substantial quantities of formaldehyde may have to be relegated to less valuable applications because of high methanol content.
Silver crystals, particularly rather small crystals such as those which pass through a 20 mesh screen (U.S. Standard) or finer, are quite effective for the purpose of attaining quite low methanol leakage during startup. However, they suffer from the disadvantage that they tend to incur higher pressure drops within a shorter time than gauze and screen or coarse sized crystals which have higher void volumes. On the other hand, it has not proved practicable merely to substitute coarser silver catalysts for the reason that coarser catalysts, especially gauze and screens, are quite slow to reach sufficient levels of catalytic activity to avoid excessive methanol leakage during the startup and first few days of continuous operation of the process.
Thus, heretofore in order to avoid excessive pressure drop over the second catalyst bed and the expense and throughput limitations which result from high pressure drop, it was necessary to shut down the process frequently to replace the use catalyst with fresh catalyst.